The present invention relates in general to processes for manufacturing gears and in particular to an improved method for forming a contour hardened gear.
In the field of gear manufacturing processes, it is known to subject gears to various metal treatment processes in order to harden the contour thereof. Such a contour hardened gear is generally characterized by a relatively thin outer layer or case which is significantly harder than the inner region or core of the gear. This thin hardened layer follows the contour of the teeth of the gear, rather than penetrating deeply therein.
The hardened outer surfaces of contour hardened gears are desirable because they are capable of sustaining relatively high contact forces applied to the faces of the teeth during use. Also, such gears can withstand relatively high bending forces experienced at the roots of the teeth. As a result, contour hardened gears are much less susceptible to premature failure resulting from fatigue. The relatively softer cores of such gears provide a measure of resiliency to absorb the shocks of these high impact forces, and further provide favorable residual compressive stress on the surfaces of the teeth.
Although the core of a contour hardened gear is somewhat softer than the hardened exterior case thereof, such core must also be hardened to a certain extent in order to support the contact and bending loads applied to the teeth. The amount of such hardening is dependent upon the intended use for the gear. Without sufficient hardening of the core of the gear, crushing and spalling may occur on the faces of the teeth, and fracturing may occur in the roots thereof.
In the past, contour hardened gears have been formed by initially prehardening an uncut gear blank (i.e., a metal annulus having no teeth formed therein) to a predetermined level of hardness determined by the desired strength of the core. Such prehardening was accomplished by raising the temperature of the gear blank in a furnace and subsequently quenching and tempering the blank at a predetermined rate and temperature. Following such initial prehardening, the uncut gear blank was machined to form a plurality of gear teeth about the circumference thereof. Lastly, the gear was subjected to a contour hardening process, typically in an inductor or coil, to provide a contour hardened gear have a sufficiently strong prehardened core. Although this process has been effective to produce contour hardened gears, it is proven to be difficult because the teeth of the gear are machined after the uncut gear blank has been prehardened, rather than when the blank is in a softened condition.